T Nation


I just read the following about fructose:

Fructose can add mass all right, fat mass. The reason fructose can make you fat is because it bypasses the enzyme phosphofructokinase-I (PFK-I), the rate-limiting step of glycolysis. This means that fructose bypasses the step which decides if a dietary sugar will be stored as glycogen or fat.
Fructose is converted into fat in the liver and most of that will end up augmenting fat cells.

Ok, this is pretty contrary to what we’ve been hearing lately. Can anyone with more biochemestry knowledge than me

Well, the above is correct. PFK-1 in muscle does not use fructose well, and on top of that, blood fructose levels are esentially zero so no fructose is getting to the muscles.

Fructose itself does not help replenish muscle glycogen,
nor does it increase insulin, and increasing insulin levels
can help with anabolism.

It is correct that fructose is metabolized in the liver
and much of it is converted to fat. Moderate amounts of
fructose don’t seem to cause problems in bodybuilding;
it’s an opinion but not a proven one perhaps that large
amounts are detrimental.

Fructose doesn’t simply “bypass” the enzyme phosphofructokinase. What, does it magically push the enzyme out of its way? This particular enzyme does not “decide” where anything goes or how it is stored, as if enzymes think. The only way fructose can increase fat storage is when there is more of “it” than there is PFK. This won’t happen, as it takes HUGE amounts to accomplish this. This has only been shown in animal models, as humans wouldn’t even consume nearly enough to “outgun” PFK. Fructose isn’t simply stored as fat, it’s first stored in muscle tissue and the liver just as any other monosaccharide. Taking in HUGE amounts,however, of any sugar, would increase fat storage. However, even then, fructose is far better than the rest because of its ability to be utlilized without the aid of insulin and because of its demonstrated thermogenic property. Whoever wrote that must have stock in a product without fructose and also needs to brush up, not only on biochem, but basic science.

The simplest answer is that it’s not so simple (as usually is the case with biochemistry). Without giving a 1hr biochem lecture, lets see what I can do.

You’re correct in that once absorbed, fructose is taken to the liver for processing. Once there, it’s incorporated into glycolytic pathways after regulatory steps (namely, it enters as fructose 1 phosphate). BUT the author of your article took a very complex topic and screwed it up in its simplification.

By bypassing regulatory steps, you’re not at all comitted to the production of fats. The fructose 1 phosphate can very easily be split into a DHAP and a glyceraldehyde (which then gets phosphorylated to form glyceraldehyde 3 phosphate). These 2 substrates then feed nicely into glycolysis and carb metabolism to be either stored as glycogen or sent to the kreb’s cycle for oxidation (it’s burnt up).

Now, this process is energy costly (requires alot of extra ATP) so that is why huge doses of fructose may affect liver function (from detoxification to blood lipoprotein alterations), but we’re talking 80, 90, 100 grams of pure fructose in a sitting.

There is another consideration that my labmate Eric Noreen and I have discussed. High fructose and glucose combined might be a no-no for fat gain and blood lipids. Since high glucose can promote the upregulation of the fat synthesis machinery and increased production of free fatty acids (FFA’s are made from extra acetylCoA in the kreb’s cycle), by adding fructose at this time the DHAP or glyceraldehyde could actually provide substrates for fat production. You see the glyceraldehyde can form glycerol and glycerol can combine with fatty acids already present (from excess acetyl coA). This makes a triglyceride that can be shipped out to the cells.

BUT REMEMBER!!! THIS HAPPENS WITH HIGH GLUCOSE TOO!!! Bottom line…do not eat huge amounts of fructose (>30g per meal is a safe bet) and do not eat alot of glucose and fructose together. The worst foods are sweetened with high fructose corn syrup and other sugars. These are the villans. Fruits and the bit of fructose in GROW are not!!!

Here is one of those instances when it is hard to reconcile basic science and biochemistry with applied research. Cy is right on many of his points. Fructose sounds great in principle. Stick a sugar in past a couple of steps of glycolysis and avoid the insulin response. First off though, for fructose to actually participate in the glycolytic pathway, it still has to be phosphorylated. Then your stuck at that darn PFK again. Second, fructose has to get into the cell. Third fructose has to get into the blood stream. The prioritization sounds backward now, but this is stream of consciousness. Anyway, when I started my Ph.D. there was a study in our lab looking at the effects of fructose based drink on high intensity aerobic performance. This is where it would have the greatest impact in theory, by increasing the rate of glycolysis, since there wouldn’t be a bunch of G-1-P or G-6-P sitting around, and conceiveably more F-6-P to push glycolysis forward. Granted if PKF is being hindered it really won’t matter. Anyway, it didn’t increase performance, or biochemical indicators of glycolysis. In fact all it increased was gas. It seems during exercise the supplement was really hard to digest and the subjects were fartin’ up a storm. It was actually ergolytic compared to a maltodextrin supplement. This really doesn’t go into the fat storage issue, just an example of the potential discrepancies between basic, animal and applied science.